1. Field of the Invention
The present invention relates to a circuit arrangement for operating gas discharge lamps, in particular high-pressure gas discharge lamps, that is used in electronic ballasts for corresponding gas discharge lamps.
High-pressure gas discharge lamps differ from low-pressure gas discharge lamps inter alia in that they require higher ignition voltages, and their colour temperature changes with the respective lamp power that is supplied. The consequence of the last-mentioned property is that high-pressure gas discharge lamps can only be dimmed with difficulty or cannot be dimmed. On the contrary, in order to maintain the colour temperature of the high-pressure gas discharge lamp, the energy that is supplied to the respective lamp must be kept constant by means of corresponding control. Accordingly, an electronic ballast for high-pressure gas discharge lamps, on the one hand, must generate a high ignition voltage and, on the other hand, must present the possibility of keeping the power supplied to the lamp constant.
2. Description of the Related Art
Known electronic ballasts for high-pressure gas discharge lamps are based on a full-bridge circuit that comprises four controllable electronic switches. This principle shall be explained in the following with reference to FIG. 4, with the circuit that is shown in FIG. 4 being known, for example, from WO-A-86/04752.
As has already been mentioned, this known circuit, for the purpose of activating a gas discharge lamp EL, in particular a high-pressure gas discharge lamp, comprises a full bridge that has four controllable switches S1-S4 which are formed, in accordance with the previously mentioned printed specification, in particular by means of bipolar transistors. Connected in the bridge branch of this full bridge there is a series resonant circuit consisting of a coil L1 and a capacitor C1, with the gas discharge lamp EL that is to be activated being arranged in parallel with the capacitor C1. The full bridge is fed with a direct voltage U0. Connected in parallel with the switches or transistors S1-S4 there are free-wheeling diodes, although these are not shown in FIG. 4 for the sake of simplicity. In order to operate the gas discharge lamp EL, in WO-A-86/04752 it is proposed that during a first operating phase the switch S4 be closed and the switches S2 and S3 be opened. Furthermore, during this first operating phase the switch S1 is switched on and off alternately at a high clock frequency. During the on-period of the switch S1 a direct current flows by way of the switch S1, the coil or inductor L1, the gas discharge lamp EL and the switch S4 which is continuously closed during this operating phase. By opening the transistor S1 the flow of current is interrupted and the magnetic energy previously built up in the coil L1 as a result of the flow of current is converted into electrical energy that supplies a counter-voltage which maintains the flow of current through the gas discharge lamp EL in the same direction until the next time the switch S1 is switched on, with the energy that is stored in the coil L1 thereby being reduced. By switching on the switch S1 again, the electric circuit previously described is closed again so that the process mentioned above is repeated. During this first operating phase in which the switches S2 and S3 are permanently open and the switch S4 is permanently closed and the switch S1 is opened and closed alternately at a high frequency, the current flows through the gas discharge lamp EL continuously in the same direction. This results in the gas discharge lamp EL flickering less during the operation thereof and a higher level of luminous efficiency is possible. During permanent operation with the direct voltage U0, however, it is possible that deposits will accumulate in the electrode region of the gas discharge lamp EL, caused by the flow of electrons flowing continuously in the same direction. In order to avoid these deposits, the polarity of the gas discharge lamp EL is repeatedly reversed at a low frequency. This is effected in that during a second operating phase the switches or transistors S1 and S4 are now permanently open and the switch S3 is permanently closed. Furthermore, during this second operating phase the switch S2 is switched on and off alternately at a high frequency so that in principle the same mode of operation ensues as that which ensues during the first operating phase previously described, although during the second operating phase the flow of current through the gas discharge lamp EL is reversed.
In summary it can accordingly be established that the full bridge shown in FIG. 4 in principle is operated with the direct voltage U0, although as a result of the low-frequency polarity-reversal between the bridge diagonals S1-S4 or S2-S3 respectively, that is, as a result of the low-frequency switch-over between the two first and second operating phases previously described, a low-frequency alternating current, the frequency of which current corresponds to the frequency of polarity-reversal, is supplied to the gas discharge lamp EL and the inductor L1. During the two operating phases, either the switch S1 or the switch S2 is switched on and off alternately at a high frequency.
The ratio of magnitudes between the clock frequency, with which the switches S1 or S2 are alternately switched on and off, and the clearly lower frequency of polarity-reversal should be selected so as to be as large as possible and can amount, for example, to 1000:1. The greater this ratio is, the smaller the dimensions of the inductor or coil L1 can be. On account of the high-frequency switch-over of the switches S1 or S2 respectively, a correspondingly high-frequency current is generated that flows through the inductor L1. The inductor serving to limit the lamp current can therefore have smaller dimensions than in the case where a low-frequency current flows through
The ignition of the gas discharge lamp EL that is shown in FIG. 4 is effected with the aid of the series resonant circuit that is formed by the inductor L1 and the capacitor C1, in which case for ignition purposes it is necessary to operate the gas discharge lamp EL at a frequency that lies close to the resonant frequency of the series resonant circuit. If this is the case, a voltage overshoot occurs at the gas discharge lamp EL leading to the ignition of the gas discharge lamp.
A similar circuit arrangement for igniting and operating a gas discharge lamp, in particular a high-pressure gas discharge lamp, is known from EP-A2-0740 492. For the purpose of igniting or operating the gas discharge lamp it is proposed in this printed specification that with the aid of a corresponding control circuit the switches S1, S4 or S2, S3 of the full bridge that are arranged in the bridge diagonals be controlled during a first operating phase in a complementary manner at a comparatively high frequency until the gas discharge lamp ignites. Subsequently, the control circuit switches over into a second operating phase (nominal operating phase) in which the control circuit activates the switches S1-S4 of the full-bridge arrangement in a complementary manner at a comparatively low frequency. Moreover, according to this printed specification a regulating device is used that is coupled on the output side by way of a capacitor to the full bridge in such a way that the full bridge is arranged parallel to the capacitor. The regulating device is used, moreover, to supply voltage to the full bridge and regulates in particular the power that is supplied to the gas discharge lamp. To this end, the voltage that is applied to the output terminals of the regulating device and also the instantaneously flowing current are measured, the corresponding values are multiplied and the actual value formed is supplied as the actual value of the lamp power of the regulating device. The control circuit previously mentioned is connected to the regulating device and presets the rated value of the output power of the regulating device, with the control circuit raising the rated value in particular during the first operating phase described above (start-up operating phase) so that the regulating device can supply a higher output power to the full bridge. The gas discharge lamp can be ignited by means of an ignition device which is coupled to the inductor L1 arranged in the bridge branch. Alternatively, the gas discharge lamp can be ignited by means of the use of the capacitor C1 that is shown in FIG. 4 and which is connected in parallel with the gas discharge lamp EL and which together with the inductor L1 forms a series resonant circuit.
A further circuit arrangement for igniting and operating gas discharge lamps, in particular highpressure metal halide gas discharge lamps, that is known from GB-A-2319 677, is shown in FIG. 5. This circuit arrangement also comprises four switches S1-S4 that are interconnected to form a full bridge and which can be formed by means of bipolar transistors or field-effect transistors. Located in the bridge branch of this full-bridge circuit there is a gas discharge lamp EL as well as a series resonant circuit formed by an inductor L1 and a capacitor C1. In order to start, that is, ignite the gas discharge lamp EL, with the aid of a corresponding control circuit which can activate the individual switches S1-S4 individually by way of corresponding bridge drivers the full bridge is operated at a comparatively high frequency which can lie in the range of 20-40 kHz. This high frequency is selected in particular in such a way that it lies close to the resonant frequency of the series resonant circuit, consisting of the inductor L1 and the capacitor C1, so that the gas discharge lamp EL ignites after a certain time. The ignition of the gas discharge lamp EL can be detected, for example, by monitoring the lamp current or by monitoring the lamp brightness. As soon as the ignition of the gas discharge lamp EL has been detected, the full bridge is switched over to a low operating frequency, which can lie in particular in the range of 50-200 Hz, in order to operate the lamp. As can be inferred from FIG. 5, moreover the circuit arrangement that is known from this printed specification comprises a transformer which is termed an ignition transformer or autotransformer, the primary winding L2 of which is arranged in series with the capacitor C1 of the series resonant circuit, whilst the secondary winding is connected in series with the gas discharge lamp EL. In the event of the occurrence of a flow of current through the capacitor C1 (this being the case in particular when the high ignition frequency is applied), this transformer with the inductors L2 and L3 is used to generate an increased voltage in the secondary coil L3 that is applied to the gas discharge lamp EL. In this way, the ignition and also the operation of the gas discharge lamp EL can be facilitated.
The circuit arrangement shown in FIG. 5 in which an autotransformer is used, the primary winding L2 of which is connected in series with the series resonant circuit capacitor C1 and the secondary winding L3 of which is connected in series with the gas discharge lamp EL, does, however, have the disadvantage that a ripple current flowing through the full bridge is also stepped up and accordingly has a negative effect upon the lamp current. Whilst the circuit arrangement that is known from EP-A2-0740 492 and which has also been discussed previously does make it possible to regulate, or keep constant, the power that is supplied to the full bridge, a comparatively large number of components are required for this so that the circuit arrangement is comparatively complex and expensive.
The underlying object of the present invention is therefore to propose an improved circuit arrangement for igniting or operating gas discharge lamps, in particular high-pressure gas discharge lamps, that avoids the problems previously described. In particular, a circuit arrangement is to be proposed which, on the one hand, makes it possible to keep the power supplied to the lamp constant, yet, on the other hand, manages to do this with fewer components than the circuit arrangement that is known from EP-A2-0740 492 and accordingly is less expensive to realize. In addition, the circuit arrangement in accordance with the invention is to enable there to be reliable ignition of the gas discharge lamp, because in particular after ignition it is to be guaranteed that the lamp current is not affected in a disturbing manner.
The object previously mentioned is achieved in accordance with the present invention by means of a circuit arrangement that in one aspect comprises four controllable switches arranged in a full bridge circuit to which a direct voltage is applied. The first switch is connected in series with the second switch and the third switch is connected in series with the fourth switch. In addition, the first switch is connected to the third switch and the second switch is connected to the fourth switch. The bridge circuit has a bridge branch in which a gas discharge lamp may be connected and which connects a nodal point between the first and second switches to a nodal point between the third and fourth switches. A control circuit is provided to switch over between first and second states at a first frequency during normal operation of the gas discharge lamp. The first state causes the first and fourth switches to be open, the second switch to be switched on and off (closed and open) at a second frequency which is higher than the first frequency, and the third switch to be closed, at least when the second switch is closed. The second state of the control circuit causes the second and third switches to be open, the first switch to be switched on and off at the second frequency and the fourth switch to be closed, at least when the first switch is also closed. The control circuit monitors a branch current flowing in the bridge branch so that whenever the branch current reaches a minimum value during the first state the control circuit closes the second switch or whenever the branch current reaches a minimum value during the second state, the control circuit closes the first switch.
In a second aspect, the control circuit is configured such that, during a warm up phase, the control circuit alternately switches over between a first state and a second state at the first frequency. During the first state, the first and fourth switches are open and the second and third switches are switched on and off together at a second frequency which is higher than the first frequency; and during the second state, the second and third switches are open and the first and fourth switches are switched on and off together at the second frequency.
In a third aspect, a series resonant circuit is coupled to the bridge branch and the control circuit is arranged to cause ignition of the gas discharge lamp. During the igniting operation, and before normal operation of the lamp, the control circuit operates the full bridge at a frequency that lies close to the resonant frequency of the series resonant circuit. Also, in this third aspect, the bridge circuit is configured such that during the igniting operation, it opens the first and second switches and switches the first and fourth switches on and off alternately at a frequency that lies close to the resonant frequency of the series resonant circuit.
Additional features of the invention are described and claimed herein. The subclaims, in each case, describe preferred and advantageous embodiments of the present invention.
According to the first aspect of the invention, the gas discharge lamp is operated in a manner analogous to the known prior art with a full-bridge circuit in such a way that there is a switch-over between the two bridge diagonals at a comparatively low frequency, with respectively the switches of the one bridge diagonal being switched on and the switches of the other bridge diagonal being switched off. In addition, at least one switch of the activated bridge diagonal is switched on and off alternately at a comparatively high frequency, with in accordance with the present invention this switch that is switched at a high frequency being closed continuously when the branch current that flows by way of the bridge branch of the full bridge has reached a minimum, that is, a lower reversal point. This switch that is switched at a high frequency can be opened at random, with the time of the opening regulating in particular the power that is supplied to the gas discharge lamp. Switching on the switch that is switched at a high frequency at the lower reversal point of the current that flows by way of the bridge branch and which in particular lies close to the current value zero makes it possible for the switch that is in each case switched at a high frequency to be treated gently since almost zero power is supplied to it at this instant. Furthermore, on account of this measure it is also possible to use, as controllable switches of the full bridge, field-effect transistors with integrated free-wheeling diodes with which a comparatively long time span is required in order to clear the electrons from the barrier layer of the respective free-wheeling diode. However, such field-effect transistors are significantly less expensive than those field-effect transistors that are also on the market and have comparatively short clearance times, such as, for example IGBT""s, so that the circuit arrangement in accordance with the invention manages with less expensive components. In accordance with a further development of the invention, the lowering of the current flowing in the bridge branch can be accelerated in part in that, in addition to the switch that is switched at a high frequency, the second switch of the activated bridge diagonal is also opened after a predetermined period of time.
The branch current flowing in the bridge branch of the full bridge is monitored in particular with the aid of an inductor which is connected into the bridge branch and which is part of a series resonant circuit that is coupled, in the bridge branch, to the gas discharge lamp that is to be controlled. The lower reversal point of the branch current can be determined by means of inductive coupling with this inductor. The instantaneous level of the branch current flowing through this inductor can be determined by way of a shunt resistor that is coupled to the full bridge.
After the ignition and before normal operation of the gas discharge lamp in accordance with a warm-up phase is effected. It is a known property of high-pressure gas discharge lamps that the latter behave in a manner that is comparatively difficult to control and in an unstable manner until they have been fully heated to the operating temperature. The full rise in temperature occurs after approximately one to two minutes. During the heating phase (with a lamp voltage of approximately 20 volts), the voltage applied to the gas discharge lamp is less than it is during normal operation (with a lamp voltage of approximately 80-90 volts). If the electronic ballast or the circuit arrangement were operated in the warm-up phase as it is during normal operation, this would result in less lamp current flowing byway of the inductor that is located in the bridge branch so that, if need be, no reliable information can be given on the branch current previously mentioned. For this reason, in accordance with the present invention it is proposed that during the warm-up phase the switches of the two bridge diagonals be switched on and off at a high frequency. At the end of the warm-up phase, a switch-over to the normal operation previously described then takes place.
In order to ignite the gas discharge lamp it is proposed that two switches that are coupled together and which are arranged in different bridge diagonals of the full bridge be opened, whilst the other switches are opened and closed alternately at a high frequency. This high frequency, at which the two switches last mentioned are alternately opened and closed, is selected in particular in such away it lies close to the resonant frequency of the series resonant circuit that is arranged in the bridge branch of the circuit arrangement in accordance with the invention so that the gas discharge lamp that is to be activated can be reliably ignited. In the case of the present invention as well an autotransformer is used, although the inductor of the series resonant circuit is used for this and the capacitor of the series resonant circuit acts on a tapping point of this inductor. In this way, the voltage that drops across the one coil half is stepped up with respect to the coil half on the lamp side so that an increased ignition voltage can be realized for the gas discharge lamp.
During the low-frequency operation of the circuit arrangement previously described, taking place, for example, at switching frequencies between 80-150 Hz, a humming noise inevitably results. The humming noise at this low frequency is comparatively quiet per se and not very disturbing. On the other hand, the harmonics that are generated by the steep edges at the switchover instants, do have a disturbing effect. For this reason, in accordance with a preferred exemplary embodiment it is proposed that these switch-over edges be configured so as to be flatter, in which case this is realized in particular by reducing the current peaks of the branch current flowing in the bridge branch before and after the switch-over. This reduction in the current peaks can be effected, for example, by adapting the control circuit of the circuit arrangement in terms of its hardware or software.